My 560SEL did this - Monovalve was shot and was stuck open. Try using clamps or vice-grips to block the hoses going to the monovalve. If the heat stops, your monovalve is bad.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280